1. Field of the Invention
The present invention relates to an active material for secondary battery, which can realize a secondary battery having excellent large current charge-discharge characteristics as well as a high energy density, and a secondary battery using the active material.
2. Description of the Related Art
As the market for portable electronic instruments such as notebook type personal computers and cellular phones is rapidly increasing, the demand for small-sized large capacity secondary batteries having high energy density, which are used in those electronic instruments, is also increasing. In correspondence to this demand, secondary batteries using an alkali metal ion such as lithium ion as the charge carrier and utilizing an electrochemical reaction associated with the charge exchange of the charge carrier have been developed. Among them, lithium ion secondary batteries are used in various electronic instruments as a large capacity secondary battery having excellent stability and high energy density.
General lithium ion secondary batteries use lithium-containing transition metal oxides as the positive electrode active material, and carbonaceous materials as the negative electrode active material. Charging/discharging is performed using the reactions for insertion and elimination of lithium ions into/from these active materials.
However, since this lithium ion secondary battery uses metal oxides having large specific gravity particularly in the positive electrode, the capacity per unit mass of the secondary battery cannot be said to be sufficient, and attempts to develop high capacity secondary batteries using lighter electrode materials have been examined.
For example, U.S. Pat. No. 4,833,048 or Japanese Patent No. 2715778 discloses a secondary battery which uses an organic compound having disulfide bonds for the positive electrode. This is to use the electrochemical oxidation-reduction reaction involving generation and dissociation of disulfide bonds, as the principle of secondary battery.
Since such secondary batteries are constituted from electrode materials containing those elements having small specific gravity, such as sulfur and carbon, as the main component, the secondary batteries show some effects in terms of providing a large capacity secondary battery with high energy density. However, the efficiency of re-combining the once dissociated bonds is low, and the stability with regard to the state of charge or state of discharge is also insufficient.
Meanwhile, as the secondary batteries using organic compounds likewise, secondary batteries using electrically conductive polymers as the electrode material have been proposed. These batteries are secondary batteries based on the reactions for electrolyte ion doping and dedoping with respect to the conductive polymer.
The doping reaction as described herein means a reaction of stabilizing excitons such as charged solitons or polarons, which are generated by oxidation or reduction of a conductive polymer, by means of couterions. On the other hand, the dedoping reaction corresponds to a reverse reaction thereof, and indicates a reaction for electrochemically oxidizing or reducing the excitons which have been stabilized by couterions.
U.S. Pat. No. 4,442,187 discloses a secondary battery which uses such a conductive polymer as the material for positive electrode or negative electrode. This secondary battery is constituted only of elements having small specific gravities, such as carbon and nitrogen, and is expected to be developed as a high capacity secondary battery.
However, in the conductive polymers, excitons that are generated by oxidation and reduction are widely delocalized over the π-electron conjugate system, and the excitons tend to interact with each other. This interaction poses limits on the concentration of the excitons generated, and thus restricts the capacity of the secondary battery. For this reason, those secondary batteries using conductive polymers as the electrode material show some effects in terms of weight reduction, but are unsatisfactory in terms of having large capacity.
Furthermore, Japanese Patent No. 3687736 discloses a secondary battery which uses a radical compound as the material participating in the electrode reaction. This secondary battery is constituted only of elements having small specific gravities, such as carbon or nitrogen. Also, since reactive unpaired electrons exist localized to radical atoms in the radical compounds, a high capacity secondary battery which allows an increase in the concentration of the reactive site can be expected. Moreover, since only the radical sites contribute to the reaction, there is provided a secondary battery having excellent stability, which has cycle properties that are not dependent on the diffusion of active material, which shows a high oxidation-reduction reaction rate because the oxidation-reduction operation is not accompanied by any change in the polymer skeleton, and in which large current charge-discharge is possible.
Nevertheless, because conventional radical materials are not conductive per se, it is necessary to add large amounts of electrical conductivity enhancers. For example, in the examples of Japanese Patent No. 3687736, powdered graphite in an amount as much as 60 mg is incorporated as a conductivity enhancer, into 30 mg of the radical compound.
Also, in a secondary battery which uses a radical material such as poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) in the electrode, since the radical material itself does not have any conductivity, a large amount of a conductive material must be added to the radical material during the preparation of the electrode paste, as shown in Japanese Patent laid open publication No. JP 2004-200059. Thus, a secondary battery having excellent output characteristics through securing of the conductivity of electrodes, and having a high energy density, cannot still be obtained.
Therefore, in order to enhance the conductivity of electrodes, various methods have been proposed. In both Japanese patent laid open publications No. JP 2005-228705 and JP 2005-209498, carbon paper was inserted between the positive electrode material and the current collector, to enhance the conductivity. Some effects were observed by this means, but there was a disadvantage that the carbon paper formed a layer of conductive material in addition to the electrode layer containing a large amount of a conventional conductive material, thus lowering the energy density.